This invention relates generally to methods of designing a fracturing treatment for a well and more particularly, but not by way of limitation, to methods of designing a schedule for flowing a proppant-laden fracturing fluid into a well.
It is well known that in developing an oil or gas well, a section of the well bore traversing the formation to be produced is sometimes subjected to a hydraulic fracturing treatment to enhance the flowability of the treated zone. In general, a fracturing treatment includes applying a fluid pressure to the geological structure of the zone until the structure is suitably broken, or fractured. Often this process is performed in three general stages: the pre-pad stage during which an ungelled, relatively thin fluid is injected into the well for conditioning the zone to be fractured; the pad stage during which a gelled fluid is injected for initiating the structural breaking; and the proppant stage during which fracturing fluid carrying proppant is injected to fracture the zone further and to keep the created fractures open once the fracturing fluid flows out.
Although the general principle of fracturing and the general stages in which a fracturing treatment is performed are well known, the specific design of a fracturing treatment job for a particular well must be determined for that well because each well has its own characteristics regarding how much fracturing fluid and proppant it will accept. What must be particularly designed is a schedule of phases having specific quantities of fracturing fluid and, for the proppant stage, specific concentrations of proppant assigned to each phase.
The design or assignment of particular quantities and concentrations for a specific treatment has in the past been to some degree as much art as science despite the monitoring of various physical parameters in the well bore. The imprecision which can arise from the "artistic" nature of such designs can result in inadequate fracturing, whereby the well does not produce as it could or whereby additional time and money are spent to perform another fracturing treatment.
Previously, some fracturing treatment jobs have been designed based upon the field experience of personnel treating other wells in the same area. More recently this experience has been supplemented by computer-aided designs utilizing various parameters related to the particular job and the particular well bore (for example, gross vertical fracture height and net vertical length or height in the well bore of the zone to be treated). A shortcoming of this type of design is that it primarily relies on the "artistic" ability of the treatment designer to assimilate the data from the other wells and to extrapolate from that data the proper specific criteria for the particular well to be treated.
A more "scientific" method of fracturing treatment design is theoretically based on several specific monitored parameters such as well bore pressure, fracture height and time-to-closure pressure. Although this technique tends to overcome the shortcoming of relying so much on the ability of the human designer required in the aforementioned prior technique, a specific implementation of this more "scientific" method (generally referred to as the "mini-frac" technique) requires pumping into the well bore a gelled test fluid that is the same composition as the fracturing fluid to be subsequently used when fracturing, and it also requires obtaining the values of several parameters from the well bore for use in the theoretical analysis. The gelled fluid necessitates additional chemicals to create the gelled fluid and the need to know the values of several physical parameters necessitates the use of wireline surveys, time and pumping, all of which cost additional money.
By way of further background and introduction to the present invention, reference is made to a pre-print of a paper to be published in the near future as SPE 15151 of the Society of Petroleum Engineers, the entire disclosure of which is incorporated herein by reference.
In view of the foregoing shortcomings of the potential for inadequate design on the one hand in the first-mentioned prior technique and the expensive theoretical design on the other hand in the second-mentioned prior technique, there is the need for an improved method of designing a fracturing treatment for a well. Such an improved method should provide a reliable design which will produce a proper fracturing treatment, but it should do so in a more economical manner than suggested by the more "scientific" prior art method. Furthermore, such an improved method should be convenient for personnel to use in the field at the well site; such use should also be capable of performance within a relatively short time to enhance further the financial economy of the improved method.